Do it Yourself.

An idea for measuring scrubber life

by Jason Rogers

From time to time, we sit around the virtual backyard and try to solve problems. Now we're going to invite you all to the virtual backyard to do some brainstorming.

Here's something we've been thinking about lately. The question has come up as a lot of guys are beefing up their factory jobbies with big manly cylinders, while the manufacturers intended them to only have little girly ones. The idea seems to be it's better that you run out of O2 before you run out of scrubber life. So the question is:

How much time is there in the scrubber?

When we're OC diving, we can measure the amount of life left in the cylinder by measuring the pressure of the gas. Up till now, the life of the scrubber has been guesswork. The variation in metabolic rate has a ratio of about 15 to 1 from maximum to minimum rate, yet scrubber life gets measured in terms of time, based on "normal" consumption, whatever that is.

Scrubber material has a rated absorption capacity. The manufacturer will have done tests to determine the amount of CO2 the material will absorb, and it's usually expressed in litres of CO2 per kilogram of absorbent. Simply work out the capacity of your scrubber in kilos, multiply that by the number of litres of CO2 each kilo will absorb. This gives you the number of litres of CO2 the scrubber can remove. Humans put out about 0.8 litres of CO2 for each litre of O2 metabolized, (it varies slightly with diet). So, divide the number of litres of CO2 your scrubber can absorb by 0.8, and you'll get the amount of O2 needed to produce a scrubberfull of CO2. Divide that amount by the water volume of your O2 cylinder, and you'll get the pressure change needed to use up your scrubber. Give yourself a goodly margin for error by subtracting 25%, and then remember that figure.

Here's how it then works in practice. Say that comes out at 100 bar. If you fill your scrubber, take note of the O2 cylinder pressure, and subtract 100. That's the cylinder pressure that will mean your scrubber is exhausted. Now if you spend a dive working hard, you can see the scrubber going down. If you're lazing around, you'll get more life out of the pack. Be sure to make corrections for dive temperature as the scrubber has less capacity in cold water.

If you do O2 flushthroughs, you'll find the "life" of the scrubber reduced. There's a way round this problem without going to an all singing all dancing electronically controlled rebreather, but I'll get to that after looking at the bells and whistles units. In a fully electronic set you can measure the amount of time the O2 add valve is open, and knowing the flow rate determine the amount of O2 being added. For example: Say the scrubber is good for 900 litres of CO2, having measured the O2 flowrate, you figure 900/0.8 (or 1125) litres of O2 takes 13500 seconds to inject. Give it a margin of say 25%, so 10125 seconds of injection is a full scrubber life. Set the unit up so that every 100 seconds of injection time takes 1% off a 100%_to_0% display. Presto! A scrubber gauge. That's if you're using time to meter the amount of O2 injected. If you're using slugging systems, the same sort of thing applies. Say each slug is 2 litres. That means that 1125/2 is the number of slugs needed to use up the scrubber. 560 slugs per scrubber pack. With a safety margin, that makes just over 400 slugs for a scrubber, so on every fourth slug, the counter takes 1 off the percentage gauge. If you don't fancy the waste of it not adding up to exactly 400 slugs, then just alter the slug volume by changing the intermediate pressure, or the size of the reservoir to make it an even 400 slugs (or 500 or 600 or whatever you decide).
Again, you may need to correct for water temperature. Consult the makers of your absorbent, they should be able to supply you with the info you need. Remember also that if you decide to fly on manual, the scrubber life display will go out. If you fly above the normal setpoint, the scrubber life will be more than the displayed life, and if you fly below the normal setpoint, the scrubber will last longer than the set thinks it will. Even though you're using a little box with flashing lights, you still have to bring a brain along on every dive.

For a manual unit with displays (the type we prefer here in the backyard) you should be using a slugger. Holding the button down long enough to keep the O2 up is a pain with a constant O2 flow at around 5 litres/minute, and you wouldn't want it any higher in case it fails open.

So, if you're running a slugger, the problem is similar to the one above. The only difference is that you need to generate the counting signals rather than tapping into the add valve opening signals. One way to do that would be to monitor the pressure in the reservoir. When it falls suddenly, that means you've opened the add valve. You don't want normal pressure switches in a high pressure O2 environment (that's really asking for it!). Instead you could use a liquid to transmit the pressure changes to a pressure switch. Have a flexible container in contact with the intermediate pressure O2. It's filled with a non corrosive, non toxic, non flammable liquid. I think glycerin might fill the bill here, it's what they fill O2 gauges with to damp needle movement. Has anyone got a safety data sheet on glycerin? Failing that, you may need to use two fluids, water for the end in contact with the O2, and oil for the end in contact with the switch.

The switch itself can be simply an oil pressure switch off a car. They conduct when the pressure is low, and open when the pressure is high. You can feed the low/high voltage to a counter, they're pretty much off the shelf items, used in everything from cars to photocopiers. You work out how many slugs the scrubber is good for, then remember that figure. As you use the set, the counter counts up, storing the information in the position of the wheels. When you refill the scrubber, just zero the counter. The beauty of it is that when you do the 6m flush, the counter only records one slug, while you're holding down the button for ages. Then it only records slugs as you maintain loop volume. This gives a much more accurate representation of how much CO2 you are producing than just looking at O2 consumption. You can also refill your O2 cylinders after every dive, and not have to track anything.

So now it's your turn. Think about what I've said here, find the faults in my logic that mean someone dies, and tell us. Scott will post the responses up here, and we can all learn. That's what the AARG is all about. We share everything we know, and hopefully the people we share info with, will let us know what they've learned. Everyone wins!

Cheers Jason =:)

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